This invention relates to red mud, a residue which results when bauxite is treated by the Bayer process, and more particularly, it relates to a process for treating red mud to recover the metal values therein.
The Bayer process is used for recovering substantially pure alumina from bauxite and uses NaOH to dissolve the alumina in the bauxite. Most of the other components of the bauxite are inert in the process, except silica, some of which reacts with the hydroxide. The inert components and the silica and silica compounds are rejected and are referred to as red mud or red sludge. This waste product from the Bayer process has presented serious disposal problems. Red mud can contain 11 to 45 wt. % Al.sub.2 O.sub.3, 5 to 66 wt. % Fe.sub.2 O.sub.3, 1 to 16 wt. % SiO.sub.2, 6 to 17 wt. % TiO.sub.2 and 3 to 8 wt. % Na.sub.2 O.
Many attempts have been made to recover the metal values from red mud. For example, U.S. Pat. No. 3,574,537 discloses a process for the treatment of red-mud to extract Fe.sub.2 O.sub.3, Al.sub.2 O.sub.3, SiO.sub.2 and Na.sub.2 O, in which SO.sub.2 is passed into a solution of red-mud and the Fe.sub.2 O.sub.3 separated therefrom. The liquor is heated until a pH of 4.5 to 5.0 is reached forming a precipitate of SiO.sub.2 and Al(OH)SO.sub.3. The precipitate is separated from the liquor which is concentrated to crystallize out Na.sub.2 SO.sub.3. Sulphuric acid is added to the separated precipitate forming water-soluble aluminum salts. SiO.sub.2 remains as a residue and is removed from solution. Water and a potassium or ammonium salt is added to the liquor from which the corresponding alum may be crystallized out.
U.S. Pat. No. 4,119,698 discloses that red mud is digested with concentrated sulfuric acid or with sulfur trioxide gas to produce sulfates that can be leached out to the resulting mass with water. The solution is then heated at a pH of 1 to precipitate titanium oxide hydrate by hydrolysis. The remaining sulfates of the solution are then obtained in solid form by evaporation, or by precipitation with acetone, and the solid is then roasted to convert the aluminum and iron to the oxide. After leaching out the sodium sulfate with water, the aluminum and iron oxide are separated by the Bayer process, which works in this case even though x-ray diffusion patterns show that the aluminum oxide is mainly .alpha.Al.sub.2 O.sub.3.
U.S. Pat. No. 4,017,425 discloses a method for activating the red mud formed in the Bayer alumina producing process for allowing its use as adsorbent, catalyst, ion-exchanging substance and clarifying substance, comprising digesting red mud and dispersing the metal oxide compound particles in the compound of metal hydroxides and silica gel.
U.S. Pat. No. 5,043,077 discloses a method of treating Bayer process red mud slurries to improve or facilitate the handling thereof, comprising adding to such a slurry a minor proportion of humic acids or humates effective to reduce the viscosity of the slurry.
Australian patent 223,794 discloses treating red mud by first calcining then forming a slag in an electric furnace followed by treating the slag with sulfuric acid. Alumina can be recovered from aluminum sulfate by heating.
German Patent 26 53 762 discloses a treatment for red mud which comprises heating to 250.degree. to 350.degree. C. (pref. 280.degree. to 320.degree. C.), adding concentrated H.sub.2 SO.sub.4 or SO.sub.3 gas, leaching the sulphates formed with water and separating the solution. Ti oxide is precipitated after adjusting the pH to 1 and the solid Ti oxide is separated by filtering. Acetone is added or the solution is evaporated to crystallize the sulphates which are roasted at 900 to 1000.degree. C. The remaining Na sulphate is leached with water and the oxides of Al and Fe remaining are treated by the Bayer process.
Japanese reference J 54137-499 discloses that silica, titania and alumina can be separated from red mud by adding sulfuric acid and then ammonia, reducing trivalent iron to divalent, and hydrolyzing by adjusting pH.
In spite of these references, there is still a great need for a process which can extract the metal values from red mud in an economical manner. The instant invention provides such a process.